The subject matter disclosed herein relates generally to neutral point clamped (NPC) power converters, and, more particularly, to a method and apparatus for electrical fault detection, identification, and protection in a neutral point clamped (NPC) power converter.
Switching mode power converters are used in many applications, including industry applications, transportation, home appliances, and power production and transmission. A common topology for a switching mode power converter is a voltage source pulse width modulated converter with two-level or multilevel output pulse width modulated voltage. Neutral point clamped (NPC) power converters have become more common due to various advantages over two-level converters. A NPC power converter includes a direct current (DC) bus capacitor comprising two series connected capacitors. The NPC power converter may further include one or more phase legs, and each phase leg may include two NPC diodes. The NPC diodes are each directly connected to the DC bus capacitor midpoint. A NPC power converter provides good utilization of semiconductor switches in the NPC power converter, requiring relatively small passive components such as input capacitor filters and output inductors, and lowered voltage stress on the output filter and load.
NPC power converters may require relatively complex of control systems for the switches in the various phase legs. Additionally, fault detection and protection is a complex process. Without proper fault detection and protection the entire NPC power converter may be destroyed during fault conditions. Failure of an NPC power converter may include blocking capability collapse of a diode or breakdown of a semiconductor switch. A result of such a failure is may be a direct short circuit across an energized power capacitor, which is followed by high fault current that may be an order of magnitude higher than the device rated current. This type of fault does not result in immediate catastrophic failure. The remaining healthy devices in the NPC power converter can sustain this operating mode for limited time, shorter than about 10 microseconds (μs). This time limit is specified by the device manufacturer and must not be exceeded to avoid total failure of the NPC power converter. If appropriate protection is not applied during the time limit, the fault will propagate through the phase leg and the entire converter.
Fault detection in an NPC power converter may be based on the de-saturation effect, which is the ability of insulated gate bipolar transistor (IGBT) switches to limit fault current. The detection is based on the fact that the IGBT switch voltage is high if the current is higher than the saturation level of the switch. The fault is detected by measuring the collector emitter voltage of the switch. However, the detection circuit may not be able to determine exact position of the faulty device, or distinguish short circuit (SC) fault of a first device from an open circuit (OC) fault of a second device. The fault case when two devices fail in short circuit may also not be able to be identified. This may result in a postponed catastrophic failure of the NPC power converter due to improper diagnosis of fault conditions.